1.1. Introduction
Scientists and researchers have been fascinated with the details of life at small scales ever since Robert Hooke saw the evidence of small structures in cork that he coined cells. This spurred the creation of the compound microscope and the quest of the late 1600s to discover how life operates beneath our very own eyes. That quest has continued even to this day as scientists look for smaller and smaller constituents that contribute to life as we know it; from proteins to functional groups, everything has an important role. The collective scientific gaze looked for finer and finer components to life, and for a short while now has focused on the prevalence of the nano world.
One hundred to one thousand times smaller than Hooke’s observed cork cells, researchers have determined that materials and features of less than 100 nm in at least one dimension can have profound effects on the behavior of cells and further tissue and organ regeneration (
Zhang and Webster, 2009). When examining nature, using nanotechnology for tissue regeneration becomes obvious. In fact, human cells create and continually interact directly with their natural nanostructured environment, called extracellular matrix (ECM). This momentous discovery spurred many researchers to attempt to more effectively mimic natural biology by creating novel nanobiomaterials and designing nanocomposite scaffolds for improved tissue and organ regenerations (
Biggs et al., 2007;
Jang et al., 2010;
Chopra et al., 2012). Decreasing material size to the nano scale dramatically increases surface roughness and the surface area to volume ratio of materials, and may lead to a higher surface reactivity and many superior physiochemical properties (i.e. mechanical, electrical, optical, catalytic, and magnetic properties) (
Zhang and Webster, 2009). The excellent properties of nanobiomaterials make them hold great potential for a wide range of biomedical applications, particularly advanced tissue/organ regeneration.
With the exponential growth in the human population and the similarly rapid increase in lifespan worldwide, there is an enormous market for various tissue and organ transplantations and engraftments. Current treatment options for damaged tissues and organs are nonideal, and often involve severe tissue/organ shortages, painful surgeries, and long recovery times without offering a complete restoration of the tissue’s and organ’s function. Simultaneously, in recent years, many health professionals have been advocating for a more active lifestyle with increased exercise and thus an increased risk of injury. These factors, and many others, put a strain on existing treatment methods and hallmark their many weaknesses. For most tissue damage caused by diseases or injuries, many current treatment methods lack the ability to restore the affected area to a level of functionality equivalent to healthy native tissue. They instead provide a stop-gap or temporary solution that either slows the progress of further degeneration or requires sacrifice of other healthy tissue (autografts). Many researchers and doctors hope that by increasing understanding of how cells and tissues interact on the nano scale and creating biomimetic nanostructured tissue constructs to better emulate natural designs, solutions that more effectively treat diseases and injuries can be discovered.
In the following sections, we will focus on the current state of nanotechnology for a series of tissue and organ regeneration. In addition, we will put special emphasis on integrating cutting-edge 3D nano/microfabrication techniques with nanobiomaterials for complex tissue and organ regeneration applications. These nanobiomaterial constituents can be made of nearly any material imaginable, including carbon nanomaterials, self-assembly nanomaterials, natural or synthetic polymers, ceramics, drug-containing spheres, or metal particles. Researchers strive to combine the appropriate nanobiomaterials, cells, and growth factors to create the ideal biomimetic tissue engineered construct that could surmount traditional methods of injury mitigation.
1.2. Nanobiomaterials for Tissue Regeneration
1.2.1. Carbon Nanobiomaterials
1.2.1.1. Carbon Nanotubes
Carbon and carbon derivatives are some of the most versatile nanomaterials that tissue engineers have in their arsenal (
Zhang et al., 2009a;
Tran et al., 2009). In addition to constituting 18% of the average human body by mass (
Frieden, 1972), carbon is a highly flexible element that can assume many nanometer-sized structures. One of the most well-explored carbon nanomaterials is carbon nanotubes (CNTs) (
Figure 1.1). CNTs have several different types, but those used in the tissue engineering field are primarily multiwalled CNTs (MWCNTs) or single-walled nanotubes (SWCNTs). They are one of the strongest materials known (
Yu et al., 2000;
Terrones, 2004), and can exhibit semiconducting (
Jung et al., 2013) and conducting (
Lan and Li, 2013) properties, making them interesting media for stimulating tissue regeneration.
Figure 1.1Comparison of carbon nanotubes and carbon nanofibers showcasing their morphological differences, and relative difference in diameter. Image is from Kim et al. (2013). One of the most prominent features of CNTs is their ability to significantly influence the electrical conductivity of scaffolds. This trait is of particular interest to groups studying tissues that rely heavily on signaling to perform functions, such as cardiac tissue. In one example, gelatin methacrylate hydrogel scaffolds modified with incorporated CNTs expressed improved cell behavior when seeded with rat cardiomyocytes. The tissue exhibited increased synchronous beating rate and a significantly lower threshold for excitation when compared to control samples without incorporated CNTs (
Shin et al., 2013). Furthermore, CNTs also tend to increase cardiomyocyte proliferation and maturation
in vitro (
Martinelli et al., 2013;
Shin et al., 2013;
Martinelli et al., 2012). Although CNTs are used for several other fields within tissue engineering, they appear to selectively steer mesenchymal stem cells (MSCs) toward a cardiac lineage when introduced into cell culture media and exposed to electrical stimulation (
Mooney et al., 2012).
Many researchers have also drawn upon the high electrical conductivity exhibited by CNTs to create conductive scaffolds for neural tissue regeneration (
Gacem et al., 2013). Improved peripheral nervous system (PNS) (
Serrano et al., 2014) and central nervous system (CNS) (
Kim et al., 2014) regeneration and stem cell performance (
Serrano et al., 2014) have been observed when utilizing CNTs. In particular, a 3D porous scaffold was fabricated from chondroitin sulfate, a biomaterial constituent of native nervous tissue, and MWCNTs via a freeze-drying method, coated with polylysine, and cultured with rat embryonic neural progenitor cells. After 20 days of culture, a viable cell population of more neuron than glial cells was observed, contrasting the 2D, CNT-less controls (
Serrano et al., 2014). In another study, MWCNTs were combined with collagen to create a collagen–CNT nanomaterial scaffold that accelerated and directed differentiation of human decidua parieltalis stem cells into a neural lineage (
Sridharan et al., 2013). The nanocomposite scaffold elucidated a previously unknown differentiation pathway of parieltalis cells, unique to this scaffold. In addition, CNTs can also be used to reveal important mechanisms of neuronal activity. In this specific study, MWCNTs with a small number of walls, dubbed “few-walled CNTs,” were used as a substrate to examine the chloride shift; a hallmark trait of neuronal disorder and injury (
Liedtke et al., 2013). Primary CNS neurons were found to...
